Investigating The Role Of ARAP3 In The Regulation Of Hematopoietic Stem and Progenitor Cells

ARAP3 is a member of the dual Arf-and-Rho GTPase-activating proteins (GAP) family, functioning specifically to inactivate its substrates Arf6 and RhoA GTPases. ARAP3 is translocated to the plasma membrane after PIP₃ binding to the first two of its five PH domains, facilitating its GAP activity in a PI3K-mediated manner. Rho family GTPases are found to play critical roles in many aspects of hematopoietic stem and progenitor cells (HSPCs), such as engraftment and migration, while a role for Arf family GTPases in hematopoiesis is less defined.

Previous studies found that either exogenous ARAP3 expression in epithelial cells or RNAi-mediated ARAP3 depletion in endothelial cells disrupts F-actin or lamellipodia formation, respectively, resulting in a cell rounding phenotype and failure to spread. This implies that ARAP3 control of Arf6 and RhoA is tightly regulated, and maintaining precise regulation of ARAP3 levels is crucial to actin organization in the cell. Although ARAP3 was first identified in porcine leukocytes, its function in the hematopoietic system is incompletely understood.

Germline deletion of Arap3 results in embryonic lethality due to angiogenic defects. Since endothelial cells are important for the emergence of HSCs during embryonic development, early lethality precludes further studying the role of ARAP3 in definitive hematopoiesis. Therefore, we generated several transgenic mouse models to manipulate ARAP3 in the hematopoietic compartment: (1) Arap3^fl/fl;Vav-Cre^tg conditional knockout mice (CKO) deletes ARAP3 specifically in hematopoietic cells, (2) Arap3^fl/fl;VE-Cadherin -Cre^tg CKO mice selectively deletes ARAP3 in embryonic endothelial cells and thereby hematopoietic cells, and (3) Arap3^{R302,3A/R302,3A} germline knock-in mice (KI/KI) mutates the first PH domain to ablate PI3K-mediated ARAP3 activity in all tissues.

We found an almost 100% and 90% excision efficiency in the Vav-Cre^tg- and VEC-Cre^tg- mediated deletion of ARAP3 in the bone marrow (BM), respectively. However, the CKO mice appear normal in steady-state hematopoiesis, showing normal peripheral blood (PB) counts and normal distributions of all lineages in the BM. Interestingly, we observed an expansion of the Lin^-Scal⁺cKit⁺ (LSK) stem and progenitor compartment in the CKO mice. This is due to an increase in the multi-potent progenitor (MPP) fraction, but not the long-term or short-term HSC (LT- or ST-HSC) fractions. Although loss of ARAP3 does not alter the frequency of phenotypically-characterized HSCs, we performed competitive BM transplantation (BMT) studies to investigate the functional impact of ARAP3 deficiency. 500 LSK cells from Arap3 CKO (Arap3^fl/fl;Vav-Cre^tg and Arap3^fl/fl;VEC-Cre^tg) or Arap3^fl/fl control littermate donors were transplanted with competitor BM cells into irradiated recipients. We observed similar donor-derived reconstitution and lineage repopulation in the mice transplanted with Arap3^fl/fl and Arap3 CKO HSCs. Moreover, Arap3 CKO HSCs show normal reconstitution in secondary transplants.

Arap3^KI/KI mice are also grossly normal and exhibit an expanded MPP compartment. Importantly, Arap3^KI/KI LSKs show impaired reconstitution compared to controls in the competitive BMT assays. Upon secondary and tertiary transplantation, reconstitution in both PB and BM diminished in the Arap3^KI/KI groups, in contrast to sustained reconstitution in the control group. Additionally, we observed a marked skewing towards the myeloid lineage in Arap3^KI/KI transplanted secondary and tertiary recipients. These data suggest a defect in HSC function in Arap3^KI/KI mice. Myeloid-skewed reconstitution also points to the possibility of selection for “myeloid-primed” HSCs and against “balanced” HSCs, as HSCs exhaust during aging or upon serial transplantation.

Taken together, our data suggest that ARAP3 plays a non-cell-autonomous role in HSCs by regulating HSC niche cells. Alternatively, the ARAP3 PH domain mutant that is incapable of locating to the plasma membrane in response to PI3K may exert a novel dominant negative function in HSCs. We are investigating mechanistically how ARAP3 controls HSC engraftment and self-renewal to elucidate the potential cell-autonomous and non-cell-autonomous roles of ARAP3 in HSCs. In summary, our studies identify a previously unappreciated role of ARAP3 as a regulator of hematopoiesis and hematopoietic stem and progenitor cell function.